Plutonium must be reduced to the metallic state for use in nuclear weapons. The chemical characteristics of plutonium metal limit the chemical processes which are operational for production scale use. Currently, there are two processes which are used for large scale conversion of plutonium oxides to metal. One method is the "bomb-reduction" method (BR), and the other is the direct oxide reduction (DOR) method. These methods suffer from major shortcomings in efficiency and economics, because they each generate significant amounts of waste by-products and require complicated recycle loops of chemical reactions to recover unreduced feedstock materials, the raw material of the chemical process. Additionally, the bomb-reduction process poses a significant problem of exposure of operating personnel to neutrons.
In the bomb-reduction (BR) method for producing plutonium metal, plutonium fluoride (PuF.sub.4), is reacted with a reducing metal, commonly a metal such as calcium, to produce the free element and halide slag; that is, Pu metal and CaF.sub.2. An alternate method for the production of plutonium metal is direct oxide reduction (DOR), in which plutonium oxide is reacted with calcium and solvent salts, to produce plutonium metal and a large volume of spent salt. An example of this is the reaction of PuO.sub.2 with Ca and CaCl.sub.2 flux to produce Pu metal and a large volume of salt, CaO.11CaCl.sub.2.
Each of these metal production methods produce waste materials which present considerable health hazards and waste handling problems. These hazards have stimulated the search for alternative methods for producing pure plutonium metal. The BR method presents human health risks due to exposure to PuF.sub.4 (alpha, neutron) neutron emission. The DOR method generates large amounts of plutonium-containing waste salts which require secondary processing. The reduction slags and residues of this process may contain as much as 5% of the plutonium contained in the original plutonium feedstock. The CaO.11CaCl.sub.2 reaction product may be dissolved with dilute mineral acids, however the acidic chloride solution from this reaction is especially corrosive and will complicate secondary recovery processes when attempted in ion exchange and solvent extraction equipment fabricated from stainless steel or ferritic alloys.
For more than 30 years, the bomb reduction method (BR) had been the only method available for large scale production of plutonium metal. This basic process includes intitation of a self-propagating exothermic reaction between PuF.sub.4 and Ca metal. The PuF.sub.4 is obtained by reacting freshly formed PuO.sub.2 with either fluorine gas or hydrogen fluoride at elevated temperature in a fluorination reactor. The charge is then placed in an insulated magnesium oxide ceramic crucible which is confined in a pressure-resistant steel vessel. Production workers operating this equipment receive significant exposure to neutron emission during this process. Upon heating and initiation, a highly exothermic reaction takes place within a few seconds. During cooling, liquid metallic plutonium collects in a pool beneath the spent reactants, which may include some remaining PuF.sub.4 feedstock. Slag dissolution and chemical recovery of unreduced plutonium fluoride is required, because the reaction rarely goes to completion. Environmental and occupational hazards associated with this process have necessitated an accelerated phase-out of bomb reduction processing for production plant use.
The direct oxide reduction process uses molten calcium as the reducing metal; plutonium oxide (PuO.sub.2) is the source of plutonium. The chemical reaction will not propagate unless the reaction by-product, CaO, is removed from the reaction site. Therefore, large quantities of CaCl.sub.2 are added as a solvent for the CaO reaction product. The reduction is performed in a magnesium-oxide crucible at 800.degree. C. in an argon atmosphere. Intense mixing is required to maintain continuous contact between the dense PuO.sub.2 and the very light Ca. The fine droplets of liquid plutonium metal which are formed gradually coalesce into a pool of plutonium metal at the bottom of the reaction vessel. The liquid-salt phase disengages from the molten plutonium phase and after cooling and solidification, it may be cleaved from the product metal.
Two problems interfere with the usefulness of the DOR method. First, there is insufficient coalescence of all of the fine plutonium droplets during the phase separation period. There may be as much as 5% of the metallic plutonium remaining in the spent salt phase, in the form of either a fine metal dispersion or as unreacted PuO.sub.2. Second, the process generates huge volumes of chloride salt residues which are difficult to process due to the corrosive nature of chloride solutions. In situ regeneration of the spent liquid salt phase with either chlorine gas or anhydrous hydrochloric acid has been used for partial recycling of the salt. Gas products of this regeneration method are severely corrosive to gas treatment facilities.
Even under optimal regeneration conditions, there are significant quantities of highly alpha-contaminated chloride salts which must be decontaminated and disposed of as the reprocessing operation proceeds. The reaction produces two moles of CaO, which must be disposed of, for every mole of PuO.sub.2 fed into the direct oxide reduction process. In addition to the large amount of waste salt which requires disposal, this salt contains significant amounts of plutonium, as the metal or unreacted oxide. There may be other actinide elements, such as americium (Am) uranium (U), or neptunium (Np), which are natural decay daughters, present in the waste salt as well.
Another problem with the DOR process is that the plutonium product is contaminated with magnesium due to liquid calcium corrosion of the MgO containers used in the process. Magnesium stabilizes plutonium metal in the delta-phase. Alpha-phase metal, which is needed for component fabrication, may be obtained by further electrorefining (ER) the product metal to purify the plutonium. While the electrorefining process provides high-quality alpha-plutonium, this slow and expensive process generates additional plutonium-contaminated salt wastes which also require chemical processing. Because the product of DOR reduction processes is typically delta-phase plutonium, the direct oxide reduction process is usually combined with an electrorefining purification step. The DOR and electro-refining (ER) processes must be combined to produce pure foundry grade plutonium metal. The consolidated reactions of metal production and metal purification makes the direct oxide reduction/electro-refining (DOR/ER) process a very expensive and time consuming method of metal production.